Numerous optical systems are used for observing a scene comprising light emitted and/or reflected from one or more target objects within the field of view of the optical system. Such optical systems typically re-direct light entering the system so as to form an image at one ore more points at which the image can be observed by an eye, or sensed by an optical sensor such as a charge coupled device (CCD).
U.S. Pat. No. 4,240,707 describes a three mirror all reflective optical system based on the classical Cooke triplet. The '707 apparatus is capable of having a moderate field of view (approximately eight to nine degrees) while still maintaining a good performance. The '707 un-obscured all reflected optical system is good for high transmission and off-axis stray light rejection for a single spectral band.
In some instances, it is desirable to observe a target object utilizing two spectral bands. Various approaches are known for achieving use of two spectral bands. Such dual channel approaches include (1) using two identical optical system, (2) implementing two relays behind an image plane, and (3) placing a dichroic beam splitter before the image plane.
Dichroic beam splitters are often used to split incident light into two separate spectral bands, a “pass band” which is transmitted through the beam splitter, and a “stop band” which is reflected by the beam splitter. By placing a dichroic beam splitter before the image plane of an optical system, light which would strike or pass through the plane first encounters the beam splitter. The beam splitter passes through/transmits a portion of that light, and reflects another portion of that light. The reflected portion is directed along a first path, the reflective channel, and the transmitted portion along a second path, the transmission channel. The light traveling through the reflective channel comprises light of a first spectral band (corresponding to the stop band of the beam splitter), and the light traveling through the transmission channel comprises light of a second spectral band (corresponding to the pass band of the beam splitter) that does not overlap the first spectral band.
Information on reflective mirror design can be found in the paper titled “Unobscured Mirror Designs”, by J. Michael Rodgers [p. 33, SPIE vol. 4832 (2002)], herein incorporated by reference in its entirety.